6.1Volume

This setting adjusts the volume of your music. Like most professional audio gear and many
consumer audio products, Rockbox uses a decibel scale where 0 dB is a reference that
indicates the maximum volume that the player can produce without possible distortion
(clipping). All values lower than this reference will be negative and yield a progressively softer
volume. Values higher than 0 dB are available and can be used to raise the volume more than
would otherwise be possible. These volume levels will ordinarily lead to distorted
sound, but might work nicely for music that has an otherwise low volume level.
The volume can be adjusted from a minimum of -89 dB to a maximum of +6 dB.
Remark: Lowering the volume below -57 dB will also affect the line-out.

6.2Bass

This setting emphasises or suppresses the lower (bass) frequencies in the sound. A value of
0 dB means that bass sounds are unaltered (flat response).

6.3Volume Limit

This setting adjusts the maximum volume of your music. The setting is by default set to the
maximum volume which equals to no limit. To set a volume limit, select a volume from
the list and the maximum volume will be limited to the selected value all over the
system.

6.4Treble

This setting emphasises or suppresses the higher (treble) frequencies in the sound. A value of
0 dB means that treble sounds are unaltered (flat response).

6.5Balance

This setting controls the balance between the left and right channels. The default, 0, means
that the left and right outputs are equal in volume. Negative numbers increase the volume of
the left channel relative to the right, positive numbers increase the volume of the right channel
relative to the left.

6.6Channels

A stereo audio signal consists of two channels, left and right. The Channels setting determines
if these channels are to be combined in any way, and if so, in what manner they will be
combined. Available options are:

Stereo.

Leave the audio signal unmodified.

Mono.

Combine both channels and send the resulting signal to both stereo channels,
resulting in a monophonic output.

Custom.

Allows you to manually specify a stereo width with the Stereo Width setting
described later in this chapter.

Mono Left.

Plays the left channel in both stereo channels.

Mono Right.

Plays the right channel in both stereo channels.

Karaoke.

Removes all sound that is common to both channels. Since most music is
recorded with vocals being equally present in both channels to make the singer
sound centrally placed, this often (but not always) has the effect of removing the
voice track from a song. This setting also very often has other undesirable effects
on the sound.

6.7Stereo Width

Stereo width allows you to manually specify the effect that is applied when the
Channels setting is set to “custom”. All values below 100% will progressively mix the
contents of one channel into the other. This has the effect of gradually centering
the stereo image, until you have monophonic sound at 0%. Values above 100% will
progressively remove components in one channel that is also present in the other. This has
the effect of widening the stereo field. A value of 100% will leave the stereo field
unaltered.

6.8Crossfeed

Crossfeed attempts to make the experience of listening to music on headphones more similar
to listening to music with stereo speakers. When you listen to music through speakers, each
ear will hear sound originating from both speakers. However, the sound from the
left speaker reaches your right ear slightly later than it does your left ear, and vice
versa.

The human ear and brain together are very good at interpreting the time differences
between direct sounds and reflected sounds and using that information to identify
the direction that the sound is coming from. On the other hand, when listening to
headphones, each ear hears only the stereo channel corresponding to it. The left
ear hears only the left channel and the right ear hears only the right channel. The
result is that sound from headphones does not provide the same spatial cues to
your ear and brain as speakers, and might for that reason sound unnatural to some
listeners.

The crossfeed function uses an algorithm to feed a delayed and filtered portion of the signal
from the right channel into the left channel and vice versa in order to simulate the spatial cues
that the ear and brain receive when listening to a set of loudspeakers placed in front of the
listener. The result is a more natural stereo image that can be especially appreciated in older
rock and jazz records, where one instrument is often hard-panned to just one of the speakers.
Many people will find such records tiring to listen to using earphones and no crossfeed
effect.

Crossfeed has the following settings:

Crossfeed.

Selects whether the crossfeed effect is to be enabled or not.

Direct Gain.

How much the level of the audio that travels the direct path from a speaker
to the corresponding ear is supposed to be decreased.

Cross Gain.

How much the level of the audio that travels the cross path from a speaker
to the opposite ear is to be decreased.

High-Frequency Attenuation.

How much the upper frequencies of the cross path audio
will be dampened. Note that the total level of the higher frequencies will be a
combination of both this setting and the Cross Gain setting.

High-Frequency Cutoff.

Decides at which frequency the cross path audio will start to
be cut by the amount described by the High-Frequency Attenuation setting.

Most users will find the default settings to yield satisfactory results, but for the
more adventurous user the settings can be fine-tuned to provide a virtual speaker
placement suited to ones preference. Beware that the crossfeed function is capable of
making the audio distort if you choose settings which result in a too high output
level.

6.9Equalizer

Figure 6.2.:

The graphical equalizer

Rockbox features a parametric equalizer (EQ). In contrast to non-parametric equalizers, a
parametric EQ enables adjusting the center frequency, gain, and width of EQ bands
separately. The ability to adjust the frequency and width of bands enables more precise
control of the EQ frequency response while avoiding the use of a large number of bands (often
12+) needed in a non-parametric EQ.

The graphic below illustrates how the width of 10kHz band can be adjusted to cover a wider
(lower Q) or narrower (higher Q) range of frequencies.

In some ways the EQ is similar to the Bass and Treble settings described earlier,
but the EQ allows you to control the sound much more carefully. Note that the
parameteric EQ bands will be applied in addition to any bass or treble tone controls.

Note: A maximum of 10 EQ bands are possible on most devices, but using more than are
required will waste battery and introduce additional rounding noise. For best results, use the
fewest number of bands required.

Rockbox’s parametric EQ is composed of up to ten different bands:

Band 0: Low shelf filter.

The low shelf filter boosts or lowers all frequencies below
a certain frequency limit, much as the “bass” control found on ordinary stereo
systems does. Adjust the “cutoff” frequency parameter to decide where the shelving
starts to take effect. For example, a cutoff frequency of 50 Hz will adjust only very
low frequencies. A cutoff frequency of 200 Hz, on the other hand, will adjust a
much wider range of bass frequencies. The “gain” parameter controls how much
the loudness of the band is adjusted. Positive numbers make the EQ band louder,
while negative numbers make that EQ band quieter. The “Q” parameter should
always be set to 0.7 for the shelving filters. Higher values will add a small boost
around the cutoff frequency that is almost always undesirable.

Bands 1-8: Peaking filters.

Peaking EQ filters boost or lower a frequency range
centered at the centre frequency chosen. Graphic equalizers in home stereos are
usually peaking filters. The peaking filters in Rockbox’s EQ lets you adjust three
different parameters for EQ bands 1 through 8. The “centre” parameter controls
the centre frequency of the frequency range that is affected as described above.
The “gain” parameter controls how much each band is adjusted, and works as
for the low shelf filter. Finally, the “Q” parameter controls how wide or narrow
the affected frequency range is. Higher Q values will affect a narrower band of
frequencies, while lower Q values will affect a wider band of frequencies.

Band 9: High shelf filter.

A high shelf filter boosts or lowers all frequencies above a
certain frequency limit, much as the “treble” control found on ordinary stereo
systems does. The high shelf filter is adjusted the same way as the low shelf filter,
except that it works on the high end of the frequency spectrum rather than the
low end.

As a general guide, EQ band 0 should be used for low frequencies, EQ bands 1 through 8
should be used for mids, and EQ band 9 should be used for highs.

Enable EQ.

This option controls whether the EQ is on or off.

Graphical EQ.

This option brings up a graphic EQ screen, which allows adjustment of
each of the three parameters described above (gain, centre frequency, and Q) for
each of the five EQ bands.

Key

Action

Middle-Right

Raises the highlighted parameter.

Middle-Left

Lowers the highlighted parameter.

Minus or
Top-Middle

Moves to the previous EQ band.

Plus or
Bottom-Middle

Moves to the next EQ band.

Centre/ Menu

Toggles the cursor among the three
parameters (gain, centre frequency,
Q) for the selected EQ band

Top-Left/ Power

Exits the graphic EQ screen.

Pre-cut.

If too much positive gain is added through the graphical EQ, your music
may distort. The Precut setting allows you to apply a global negative gain to
decoded audio, cancelling out positive gain from the EQ. This will prevent
distortion when boosting certain frequency ranges, at the expense of making audio
quieter.

Alternatively, precut can be used with a flat EQ curve to implement a volume cap. For
example, on a player that allows overdriving the headphone output to +6dB, maximum
volume can be capped to +0dB by applying 6dB of precut. Note that precut is not
applied if EQ is disabled.

Simple EQ.

This option provides an easier alternative for those who are daunted by all of the
parameters that can be adjusted using the graphical EQ. With the Simple EQ, the only
parameter that can be adjusted is the gain.

Advanced EQ.

This sub menu provides options for adjusting the same parameters as the
Graphical EQ. The only difference is that the parameters are adjusted through textual
menus rather than through a graphic interface.

Save EQ Preset.

This option saves the current EQ configuration in a .cfg file.

Browse EQ Presets.

This menu displays a list of EQ presets, as well as any EQ
configurations saved using the Save EQ Preset option. Users unfamiliar with the
operation of a parametric EQ may wish to use the presets instead of trying
to configure the EQ, or use the presets for designing their own custom EQ
settings.

6.10Dithering

This setting controls the dithering and noise shaping functionality of Rockbox.

Most of Rockbox’ audio file decoders work at a higher bit depth than the 16 bits used for
output on the player’s audio connectors. The simplest way to convert from one bit depth
to another is simply discarding all the surplus bits. This is the default behaviour,
and adds distortion to the signal that will vary in character along with the desired
sound.

Dithering adds low-level noise to the signal prior to throwing away the surplus bits, which
gives the resulting signal a uniform noise floor which is independent of the signal. Most
people find this noise preferable to the time-varying noise heard when not performing
dithering.

After dithering, noise shaping is performed. This basically just pushes the dithering noise to
the parts of the frequency spectrum humans cannot hear so easily. In Rockbox’ case, some of
the noise is pushed up to above 10 kHz.

This setting will be put to its best use when listening to dynamic music with frequently
occuring quiet parts, classical music being a typical example. It is worth noting that the effects
of dithering and noise shaping are very subtle, and not easily noticable.

Rockbox uses highpass triangular distribution noise as the dithering noise source, and a third
order noise shaper.

6.11Timestretch

Enabling Timestretch allows you to change the playback speed without it affecting the pitch of
the recording. After enabling this feature and rebooting, you can access this via the Pitch
Screen. This function is intended for speech playback and may significantly dilute your
listening experience with more complex audio. See section 4.3.3 for more details about how to
use the feature.

6.12Haas Surround

This setting implements the Haas effect with adjustable delay time to enhance the stereo effect
of the sound. A full range Haas effect creates the impression that sound starts from one
channel and ends in the other. Therefore, four additional functions are provided to move the
stage back to the center: Balance to change the left-right channel output ratio. A
bypass band for frequencies that mostly contain vocals, using f(x1), and f(x2) to set
frequencies which are not affected. The SIDE ONLY setting uses mid-side processing
to determine and apply effect to the side channel only. Finally, the Dry/Wet Mix
setting adjusts the proportion mixed from the original (dry) and ’effected’ (wet)
signals.

6.13Perceptual Bass Enhancement

This setting implements a group delay correction and an additional biophonic EQ to
emphasize to boost bass perception. The precut setting provides negative overall gain to
prevent possible audio distortion due to the EQ gain. The defult precut value is set to -2.5 dB
and can be adjust from 0 dB to -4.5 dB. Note that this effect will stack with any other EQ
applied.

6.14Auditory Fatigue Reduction

Human hearing is more senstive to some frequency bands. This setting applies additional
equalization and bi-shelf filtering to reduce signals in these bands to minimize the chance that
temporary threshold shift (auditory fatigue) occurs.

6.15Compressor

The Compressor reduces, or compresses, the dynamic range of the audio signal. This makes
the quieter and louder sections closer to the same volume level by progressively reducing the
gain of louder signals. When subsequently amplified, this has the effect of making the quieter
sections louder while keeping the louder sections from clipping. This allows listening to the
quiet sections of dynamic material in noisy environments while preventing sudden loud
sections from being overbearing.

There are several settings associated with the compressor. The first, and most important, is
the Threshold. The threshold is the audio input level at which the compressor begins to act.
Any level louder than the threshold will be compressed to some extent. The maximum amount
of compression, or the quietest level at which the compressor will operate, is -24 dB. The
default of Off disables the compressor.

The Makeup Gain setting has two options: Off and Auto. Off means that the compressed
audio will not be amplified after compression. The default of Auto will amplify the signal so
that the loudest possible signal after compression will be just under the clipping limit. This is
desirable because the compressed signal without makeup gain is quieter than the input signal.
Makeup Gain in Auto restores the signal to the maximum possible level and brings the quieter
audio up with it. This is what makes it possible to hear the quieter audio in noisy
environments.

The Ratio setting determines how aggressively the compressor reduces gain above the
threshold. For example, the 2:1 setting means that for each two decibels of input signal above
the threshold, the compressor will only allow the output to appear as one decibel. The higher
the ratio, the harder the signal is compressed. The ratio setting of Limit means essentially a
ratio of infinity to one. In this case, the output signal is not allowed to exceed the threshold at
all.

The Knee setting determines how abrupt the transition is from a non-compressed signal to a
compressed signal. Hard Knee means that the transition occurs precisely at the threshold. The
Soft Knee setting smoothes the transition from plus or minus three decibels around the
threshold.

The Attack Time setting sets the delay in milliseconds between the input signal exceeding the
activation threshold and acting upon it.

The Release Time setting sets the recovery time after the signal is compressed. Once the
compressor determines that compression is necessary, the input signal is reduced
appropriately, but the gain isn’t allowed to immediately return to normal levels. This is
necessary to reduce artifacts such as “pumping.” Instead, the gain is allowed to return
to normal at the chosen rate. Release Time is the time for the gain to recover by
10 dB.